Certain systems require highly accurate temperature measurements for proper operation. For example, a space launch vehicle may include hundreds of temperature sensors for accurate monitoring of a variety of subsystems for proper functioning. In a specific example, a launch vehicle with a liquid propellant rocket engine may require accurate monitoring of the temperature of cryogenic propellant. The launch vehicle may have a control unit for thermal management of the cryogenic propellant. The control unit may require temperature measurements of the cryogenic propellant at different locations on the launch vehicle such as on propellant tanks, fluid conduits, valves, and other propellant system components. In some examples, the control unit may be located a relatively long distance from where temperature measurements are taken.
A resistance temperature detector (RTD) is a device containing an RTD sensor (e.g., a resistance element) which may be mounted to a component for which temperature measurements are desired. Passing a small amount of electrical current (i.e., an excitation current) through the RTD sensor generates a voltage across the RTD sensor. The voltage across the RTD sensor is used to determine the resistance of the RTD sensor at the time when the current is passed through the RTD sensor. The sensor resistance is used to determine the temperature of the RTD sensor which varies linearly with sensor resistance. In this regard, the hotter the RTD sensor becomes, the higher the sensor resistance, and vice versa. The RTD sensor may be electrically coupled to a control unit which may measure the sensor voltage for determining the sensor resistance for subsequent correlation to sensor temperature.
In a conventional two-wire RTD, the RTD sensor may be electrically coupled to the control unit by a pair of wire leads such as insulated copper wires. Unfortunately, the resistance of the wire leads adds to the resistance of the RTD sensor, such that the total electrical resistance determined by the control unit is greater than the resistance of the RTD sensor alone, resulting in an erroneous temperature measurement. The error in temperature measurement is proportional to the length of the wire leads such that very long wire leads introduce correspondingly large temperature measurement errors. In addition, the resistance of the wire leads is different at different temperatures. For example, when conventional two-wire RTDs are used at cryogenic temperatures (e.g., less than −300 F) such as for monitoring a cryogenic propellant system of a launch vehicle, the temperature measurement errors may be relatively large due to relatively small changes in resistance of the wire leads at cryogenic temperatures.
Attempts to compensate for errors associated with conventional two-wire RTDs include adjusting the total amount of resistance at the control unit by an amount equal to the static resistance of the wire leads. The static resistance of the wire leads can be calculated based on known resistance-per-foot values of each wire lead at a given temperature. Alternatively, the static resistance of wire leads at a given temperature can be measured. Unfortunately, during a flight or mission, the actual resistance of the wire leads may be different than the calculated or measured resistance of the wire leads. Furthermore, some lengthwise sections of the wire leads may be colder or hotter than the temperatures for which the static resistance was calculated or measured.
Other attempts to compensate for errors associated with conventional two-wire RTDs include the development of three-wire RTDs and four-wire RTDs. A three-wire RTD adds a third wire lead to a standard two-wire RTD. The third wire lead is used to transmit to the control unit a feedback signal which the control unit uses to compensate for temperature measurement errors caused by the added resistance of the wire leads. However, for a space launch vehicle having hundreds of temperature sensors, the addition of a third wire lead to the RTDs adds to the cost and weight of the launch vehicle and detracts from vehicle performance. For example, the increased weight of three-wire RTDs may result in a reduction in payload capability and/or available propellant mass of the launch vehicle. A four-wire RTD adds two wire leads to a standard two-wire RTD and simplifies the measurement process by only requiring a single voltage measurement by the control unit. The two extra wires do not carry the excitation current and therefore do not contribute to the measurement error. However, the two additional lead wires of a four-wire RTDs further increase the cost and weight of the RTD which further detracts from vehicle performance.
As can be seen, there exists a need in the art for a lightweight, low-cost resistance temperature detector capable of providing highly accurate temperature measurements.